1. Technical Field
The present invention relates to an optical position measuring instrument.
2. Background Information
From the second embodiment described in US Patent Application Publication No. 2012/105862 A1 of the present Applicant, the entire contents of which are incorporated herein by reference, an optical position measuring instrument is known which includes two scales movable relative to one another. A first scale, which in the aforementioned document is called a scanning grating, includes a first graduation embodied as an incident light grating. A second scale, which in that document is called an index disk, includes a second graduation, which includes a radial transmitted light grating and a reflector. The first scale and the second scale are disposed movable relative to one another; the two scales are movable independently of one another. The first scale is disposed movable in a first linear measuring direction; the second scale is disposed rotatably about an axis of rotation. With the aid of this position measuring instrument, position signals with regard to the position of the respective scale or scales are generated, which in one possible embodiment are delivered to a downstream evaluation unit, which by way of them controls the positioning of machine components.
To make the use of such a position measuring instrument in machines for high-precision applications possible, a scale-dependent position correction table is used in the measurement operation for correcting the typically present short-range grating errors in the graduations employed. Such a position correction table is created, for instance, in the manufacture of the corresponding graduations, or in the calibration of the machine. The correction data stored in the table are then used in the measurement operation for correcting the position signals generated. To enable correcting the incident grating errors in this way, the particular position correction table must be put in relation to the associated graduation; this is hereinafter called referencing. Such referencing requires that the position of a scanning beam be detected by measurement at a defined reference position on the graduations employed, so that in the measurement operation the particular position correction table that is referenced to the reference position can be used. The corresponding reference position is typically predetermined by one or more markings or reference markings on the respective scale. If such a reference marking is integrated into the graduation, however, the result is an unwanted impairment of the position signals.
In the case of the optical position measuring instrument known from US Patent Application Publication No. 2012/105862 A1, with scales that can be positioned independently of one another, such referencing must be done for each of the two graduations used.
Besides the referencing in the case where a position correction table is used, referencing can also be required particularly in incremental position measuring instruments, in order that, at a known position along the measurement path, an absolute reference to the incremental measurement can be established. The aforementioned problems can arise in this kind of referencing as well.
Hereinafter, the term “referencing” is meant to cover both of the variant procedures explained above. That is, both detecting the position of a scanning beam by measurement at a defined reference position on the graduations employed, and the establishment of an absolute positional reference in the case on an incremental measurement.